Dirty buffer pages after issuing CHECKPOINT

I am currently working on a test system and due to the nature of the queries I want to optimise, I am trying to simulate a "cold" read a well as I can. Part of that is clearing the buffer cache before performing the queries. From everything I can find dirty buffer pages are supposed to be written during a checkpoint. However, even after issuing a CHECKPOINT, there still seem to be 169 dirty pages of my database in the buffer pool (assessed via SELECT * FROM sys.dm_os_buffer_descriptors WHERE database_id=7 AND is_modified=1). Is there anything I am misunderstanding about checkpoints or the content of sys.dm_os_buffer_descriptors? If not, why do I still have dirty pages after they were supposedly written away?